Thermodynamic scaling of relaxation: insights from anharmonic elasticity.

Using molecular dynamics simulations of a molecular liquid, we investigate the thermodynamic scaling (TS) of the structural relaxation time [Formula: see text] in terms of the quantity [Formula: see text], where T and ρ are the temperature and density, respectively. The liquid does not exhibit strong virial-energy correlations. We propose a method for evaluating both the characteristic exponent [Formula: see text] and the TS master curve that uses experimentally accessible quantities that characterise the anharmonic elasticity and does not use details about the microscopic interactions. In particular, we express the TS characteristic exponent [Formula: see text] in terms of the lattice Grüneisen parameter [Formula: see text] and the isochoric anharmonicity [Formula: see text]. An analytic expression of the TS master curve of [Formula: see text] with [Formula: see text] as the key adjustable parameter is found. The comparison with the experimental TS master curves and the isochoric fragilities of 34 glassformers is satisfying. In a few cases, where thermodynamic data are available, we test (i) the predicted characteristic exponent [Formula: see text] and (ii) the isochoric anharmonicity [Formula: see text], as drawn by the best fit of the TS of the structural relaxation, against the available thermodynamic data. A linear relation between the isochoric fragility and the isochoric anharmonicity [Formula: see text] is found and compared favourably with the results of experiments with no adjustable parameters. A relation between the increase of the isochoric vibrational heat capacity due to anharmonicity and the isochoric fragility is derived.

Thermodynamic scaling of vibrational dynamics and relaxation.

We investigate by thorough molecular dynamics simulations the thermodynamic scaling (TS) of a polymer melt. Two distinct models, with strong and weak virial-energy correlations, are considered. Both evidence the joint TS with the same characteristic exponent γts of the fast mobility-the mean square amplitude of the picosecond rattling motion inside the cage-and the much slower structural relaxation and chain reorientation. If the cage effect is appreciable, the TS master curves of the fast mobility are nearly linear, grouping in a bundle of approximately concurrent lines for different fragilities. An expression of the TS master curve of the structural relaxation with one adjustable parameter less than the available three-parameter alternatives is derived. The novel expression fits well with the experimental TS master curves of thirty-four glassformers and, in particular, their slope at the glass transition, i.e., the isochoric fragility. For the glassformer OTP, the isochoric fragility allows to satisfactorily predict the TS master curve of the fast mobility with no adjustments.

Anisotropy of the monomer random walk in a polymer melt: local-order and connectivity effects.

The random walk of a bonded monomer in a polymer melt is anisotropic due to local order and bond connectivity. We investigate both effects by molecular-dynamics simulations on melts of fully-flexible linear chains ranging from dimers (M = 2) up to entangled polymers (M = 200). The corresponding atomic liquid is also considered a reference system. To disentangle the influence of the local geometry and the bond arrangements, and to reveal their interplay, we define suitable measures of the anisotropy emphasising either the former or the latter aspect. Connectivity anisotropy, as measured by the correlation between the initial bond orientation and the direction of the subsequent monomer displacement, shows a slight enhancement due to the local order at times shorter than the structural relaxation time. At intermediate times-when the monomer displacement is comparable to the bond length-a pronounced peak and then decays slowly as t (-1/2), becoming negligible when the displacement is as large as about five bond lengths, i.e. about four monomer diameters or three Kuhn lengths. Local-geometry anisotropy, as measured by the correlation between the initial orientation of a characteristic axis of the Voronoi cell and the subsequent monomer dynamics, is affected at shorter times than the structural relaxation time by the cage shape with antagonistic disturbance by the connectivity. Differently, at longer times, the connectivity favours the persistence of the local-geometry anisotropy, which vanishes when the monomer displacement exceeds the bond length. Our results strongly suggest that the sole consideration of the local order is not enough to understand the microscopic origin of the rattling amplitude of the trapped monomer in the cage of the neighbours.

Cage effect in supercooled molecular liquids: Local anisotropies and collective solid-like response.

Both local geometry and collective extended excitations drive the moves of a particle in the cage of its neighbours in dense liquids. The strength of their influence is investigated by the molecular dynamics simulations of a supercooled liquid of fully flexible trimers with semirigid or rigid bonds. The rattling in the cage is investigated on different length scales. First, the rattling anisotropy due to local order is characterized by two order parameters sensing the monomers succeeding or failing to escape from the cage. Then the collective response of the surroundings excited by the monomer-monomer collisions is considered. The collective response is initially restricted to the nearest neighbours of the colliding particle by a Voronoi analysis revealing elastic contributions. Then the long-range excitation of the farthest neighbours is scrutinised by searching spatially extended correlations between the simultaneously fast displacements of the caged particle and the surroundings. It is found that the longitudinal component has stronger spatial modulation than the transverse one with a wavelength of about one particle diameter, in close resemblance with experimental findings on colloids. It is concluded that the cage rattling is largely affected by solid-like extended modes.

Weak links between fast mobility and local structure in molecular and atomic liquids.

We investigate by molecular-dynamics simulations, the fast mobility-the rattling amplitude of the particles temporarily trapped by the cage of the neighbors-in mildly supercooled states of dense molecular (linear trimers) and atomic (binary mixtures) liquids. The mixture particles interact by the Lennard-Jones potential. The non-bonded particles of the molecular system are coupled by the more general Mie potential with variable repulsive and attractive exponents in a range which is a characteristic of small n-alkanes and n-alcohols. Possible links between the fast mobility and the geometry of the cage (size and shape) are searched. The correlations on a per-particle basis are rather weak. Instead, if one groups either the particles in fast-mobility subsets or the cages in geometric subsets, the increase of the fast mobility with both the size and the asphericity of the cage is revealed. The observed correlations are weak and differ in states with equal relaxation time. Local forces between a tagged particle and the first-neighbour shell do not correlate with the fast mobility in the molecular liquid. It is concluded that the cage geometry alone is unable to provide a microscopic interpretation of the known, universal link between the fast mobility and the slow structural relaxation. We suggest that the particle fast dynamics is affected by regions beyond the first neighbours, thus supporting the presence of collective, extended fast modes.

Competition of the connectivity with the local and the global order in polymer melts and crystals.

The competition between the connectivity and the local or global order in model fully flexible chain molecules is investigated by molecular-dynamics simulations. States with both missing (melts) and high (crystal) global order are considered. Local order is characterized within the first coordination shell (FCS) of a tagged monomer and found to be lower than in atomic systems in both melt and crystal. The role played by the bonds linking the tagged monomer to FCS monomers (radial bonds), and the bonds linking two FCS monomers (shell bonds) is investigated. The detailed analysis in terms of Steinhardt's orientation order parameters Ql (l = 2 - 10) reveals that increasing the number of shell bonds decreases the FCS order in both melt and crystal. Differently, the FCS arrangements organize the radial bonds. Even if the molecular chains are fully flexible, the distribution of the angle formed by adjacent radial bonds exhibits sharp contributions at the characteristic angles θ ≈ 70°, 122°, 180°. The fractions of adjacent radial bonds with θ ≈ 122°, 180° are enhanced by the global order of the crystal, whereas the fraction with 70° ~/< θ ~/< 110° is nearly unaffected by the crystallization. Kink defects, i.e., large lateral displacements of the chains, are evidenced in the crystalline state.

Comparative evaluation of the group of teeth with the best prediction value in the mixed dentition analysis.

AIM: Mixed dentition analysis allows the prediction of the tooth width of non erupted permanent canines and premolars (SPCP). It is an essential factor in treatment planning since it could predict a tooth-size to arch-length discrepancy. The aim of this study was to determine which group of permanent teeth presented the best prediction capability for the width sums of the unerupted canine and premolars in an Italian sample. MATERIALS AND METHODS: A total of 150 subjects with complete permanent dentition were selected. Their dental casts were measured to 0.01 mm with a digital caliper. Pearson correlation and T-test were used for the statistical analysis. RESULTS/STATISTICS: The combination of the sums of the four lower incisors and the first upper molars showed the best prediction capability for the SPCP in the Italian sample. Moreover new regression equations were formulated including sex and arch as additional predictor variables. In 80% of the cases estimated differences between the predicted and the actual values were smaller than 1 mm. CONCLUSION: It can be concluded that the new regression equations proposed seemed to show good accuracy.

Fetal lung maturity evaluation with fluorescence polarization of the amniotic fluid.

Fluorescence polarization of the amniotic fluid from 39 high risk pregnancies requiring preterm delivery was measured in order to assess the maturity of the fetal lung. The study population included 15 cases of intrauterine growth retardation, ten maternal hypertension, five maternal Hodgkin's disease, three placenta previa, two fetal malformation, two polyamnios, one untreated diabetes, one maternal nephropathy. All patients underwent a single amniocentesis before deciding whether to deliver a preterm baby and FP of the amniotic fluid was done within two hours from amniocentesis. In five cases this was > 0.311, the cut-off limit taken as an indicator of fetal pulmonary status, and three of these developed respiratory distress syndrome. In 34 cases FP values were < or = 0.311; in spite of the apparent lung maturity two of these newborns developed respiratory distress syndrome. On the basis of these results the FP sensitivity was calculated as 60%, specificity 94% and the overall accuracy 90%.

[Enoxacin in the treatment of bacterial infections of the respiratory system]. / L'enoxacina nel trattamento di infezioni batteriche a carico dell'apparato respiratorio.

The above study was carried out in 30 patients, 22 males, 8 females, aged between 30 and 66 years, average age 57.17 +/- 1.43 years, 19 of whom with flare-ups of chronic or asthmatic bronchitis, 6 suffering from flare-ups of chronic bronchoectasic bronchitis, and 5 with acute tracheobronchitis. Treatment with two 300 mg tablets of enoxacin daily lasted from 10 to 15 days. The clinical result was good in 95% of the cases, and microbiological results were also satisfactory in that the micro-organisms isolated were sensitive to the antibiotic. Tolerance was good both locally and generally, only two patients complained of slight stomach discomfort. Both symptoms and laboratory findings improved progressively, and the result was considered excellent in four cases, good in 25 and only fair in one.

Mechanism of polyamine inhibition of a leaf protease.

The inhibition of a highly purified alfalfa (Medicago sativa) leaf protease by naturally occurring polyamines is reported. The tetraamine spermine shows the highest inhibitory effect, with the maximum inhibition at 0.1 mM. Kinetic data indicate an apparent hyperbolic competitive inhibition. CD measurements show that in the presence of 0.1 mM spermine the enzyme undergoes a conformational change with the loss of 16% alpha-helix secondary structure content. Both the inhibition and the conformational change are prevented by high ionic strength. These data suggest a novel control mechanism of proteolytic activity in the leaf.

Purification, stability and kinetic properties of highly purified adenosine deaminase from Bacillus cereus NCIB 8122.

Adenosine deaminase (adenosine aminohydrolase, EC 3.5.4.4) from Bacillus cereus NCIB 8122 has been purified to electrophoretic homogeneity by ammonium sulfate precipitation, gel filtration through Sephadex G-100, DEAE-Sephadex A-50 chromatography and ion-exchange HPLC on DEAE-Polyol. The enzyme activity is stabilized (at temperatures from 0 degrees C to 40 degrees C) by 50 mM NH4+ or K+, while it is irreversibly lost in the absence of these or a few other monovalent cations. Glycerol (24% by volume) helps the cation in stabilizing the enzyme activity above 40 degrees C, but also exerts per se a noticeable protecting effect at room temperature. B. cereus adenosine deaminase displays the following properties: Mr on Sephadex G-200, 68,000; Mr in SDS-polyacrylamide gel electrophoresis, 53,700; optimal pH-stability (in the presence of 50 mM KCl) over the range 8-11 at 4 degrees C, and maximal catalytic activity at 30 degrees C between pH 7 and 10; Km for adenosine around 50 microM over the same pH range and Km for 2'-deoxyadenosine around 400 microM.